CN119403633A

CN119403633A - Ultrasonic transducers for high temperature applications

Info

Publication number: CN119403633A
Application number: CN202380048871.8A
Authority: CN
Inventors: 弗雷德里克·纳瓦基亚; 阮党奇; 居伊·弗亚尔; 路易斯·帕斯卡·陈胡休
Original assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA; Institut National des Sciences Appliquées Centre Val de Loire
Current assignee: Commissariat a lEnergie Atomique et aux Energies Alternatives CEA; Institut National des Sciences Appliquées Centre Val de Loire
Priority date: 2022-06-23
Filing date: 2023-06-21
Publication date: 2025-02-07
Also published as: FR3137252B1; EP4543601A1; JP2025520741A; FR3137252A1; KR20250027262A; WO2023247639A1

Abstract

The invention relates to an acoustic transducer device comprising-a piezoelectric transducer (10) between a front electrode (11) and a rear electrode (12) and made of piezoelectric material, -a front aperture (15) arranged such that the front electrode is arranged between the piezoelectric material and the front aperture (15), -the device is configured to emit sound waves (EW) to the front aperture or to detect sound waves (RW) propagating from the front aperture, -the device comprises a rear part (13) against or forming the rear electrode, the rear part forming an acoustic damping backing of the device, the device being characterized in that the rear part comprises a porous metal with a melting point higher than 200 ℃.

Description

Ultrasound transducer for high temperature applications

Technical Field

The present invention relates to an acoustic transducer intended for use in operations such as nondestructive detection, obstacle detection, ranging, etc. in environments that may be under high temperature and high pressure, such as in nuclear power plants.

Background

Ultrasonic non-destructive testing is well suited for monitoring structures to track aging resistance and the appearance of any imperfections, or for ranging or obstacle detection operations. Some applications are performed in a high temperature environment or a high pressure environment. This is the case, for example, in the field of aircraft engines, or in the petroleum industry, or in nuclear reactors.

Fig. 1A schematically illustrates a prior art ultrasound transducer 1 _AA intended for use in a high temperature environment. High temperatures are generally understood to be temperatures above 200 ℃ or above 600 ℃. The active elements of the transducer are arranged in the housing 2. The active element includes a piezoelectric transducer 10 formed of a piezoelectric material, the piezoelectric transducer 10 being interposed between a front electrode 11 and a rear electrode 12. Each electrode is connected to a circuit 20. The front electrode is arranged facing an inlet aperture 15 formed in the housing 2. Such a construction is described in US 2014215784.

Under the action of alternating voltage applied between the electrodes, the piezoelectric transducer generates acoustic wave EW. The emitted sound wave EW propagates through the orifice 15 to the external medium 3 outside the housing 2. The transducer may comprise a plate 14 to match the acoustic impedance between the transducer and the external medium 3. Whereby the transducer operates in a transmit mode (transmission mode).

The transducer may also be operated in a receiving mode in which the acoustic wave RW propagates from the external medium 3 to the transducer. The received sound wave RW causes vibration of the piezoelectric transducer. This results in an alternating voltage between the terminals of the electrodes 11 and 12. Whereby the transducer operates in a receive mode.

For high temperature applications, the piezoelectric material may be lithium niobate, as described in patent US 9425384.

Fig. 1B shows a timing diagram of the vibration amplitude of a prior art electrical transducer after receiving sound. The y-axis corresponds to the amplitude of the vibration wave of the piezoelectric transducer measured by the circuit, while the x-axis corresponds to time. The emission is triggered by applying an alternating voltage of a few microseconds to a few tens of microseconds. However, the duration of vibration of the piezoelectric transducer is much longer, on the order of hundreds of μs (microseconds). This can lead to reduced performance of the transducer.

To solve this problem, it is known to connect the rear electrode with an element to form a "vibration damping backing (damping backing)", commonly referred to as a "backing element". The primary function of the vibration damping backing is to dampen the vibrations of the assembly formed by the piezoelectric transducer and the electrode at the rear electrode. In the prior art, the vibration damping backing of the transducer may be formed of a polymer doped with high density particles, such as tungsten or lead particles. The expression "particle composite" is also used. The random distribution of particles causes multiple reflections and the acoustic wave decays due to destructive interference. This makes it possible to reduce the duration of the acoustic pulse of the transducer. However, such composites are not suitable for high temperature applications.

Publication "Porous ceramics as backing element for high temperature transducers",IEEE Transactions on Ultrasonics,Ferroelectrics and Frequency Control, volume 62, 12, pages 360-372, 2015 describes a transducer intended for use at high temperatures. The transducer includes a vibration damping backing connected to the electrode and made of porous ceramic. The use of ceramics makes it possible to achieve compatibility with implementations at high temperatures. However, manufacturing such transducers requires bonding the ceramic forming the vibration damping backing to the electrode. There is uncertainty in the durability of such joints. The resistance of porous ceramics to exposure to high radiation is also uncertain.

WO2016/124941 describes a device for emitting sound waves comprising a backing element formed from a metal foam.

The inventors developed an ultrasound transducer that was intended to be used under high radiation and at high temperatures or exposed to high temperature gradients for long run times (i.e., events over several years or even decades).

Disclosure of Invention

A first object of the present invention is an acoustic transduction apparatus comprising:

-a piezoelectric transducer formed of piezoelectric material interposed between the front electrode and the rear electrode;

-a housing accommodating the piezoelectric transducer, the front electrode and the rear electrode;

-a front aperture formed in the housing and arranged such that the front electrode is arranged between the piezoelectric material and the front aperture;

The device is configured to emit sound waves to the front aperture or detect sound waves propagating from the front aperture;

the device comprises a rear member against or forming the rear electrode, the rear member forming a backing element of the device, the device being characterized in that the rear member is a porous metal material having a melting point higher than 200 ℃.

The device may have one of the following features, implemented alone or in technically feasible combinations.

The rear part forms a rear electrode.

The melting point of the metallic material is higher than 600 ℃.

The piezoelectric material has a curie temperature, and the melting point of the metal material is higher than the curie temperature of the piezoelectric material;

the curie temperature of the piezoelectric material is higher than 1000 ℃.

The volume fraction of pores of the porous metal material is 20% to 60%, or 25% to 50%, or 25% to 40%.

The pores are filled with air.

The average size of the pores, which corresponds to the average diameter of each pore, is less than 100 μm.

The piezoelectric material is selected from lithium niobate and barium titanate.

The metallic material comprises at least one element selected from Ni, fe, pd, ag, au, cu, pd, al.

The metallic material is a stainless steel alloy.

Another object of the invention is to transmit or receive sound waves using a device according to the first object of the invention, which device is arranged in a medium with a temperature above 200 ℃ through an orifice.

The invention will be better understood from a reading of the disclosure of examples of embodiments presented in the remainder of the specification in combination with the figures listed below.

Drawings

Fig. 1A schematically shows a transducer according to the prior art.

Fig. 1B illustrates the vibration of the transducer.

Fig. 2A shows a first embodiment of a transducer according to the invention.

Fig. 2B shows a second embodiment of a transducer according to the invention.

Fig. 3A schematically illustrates poor impedance matching between a piezoelectric transducer and a component bonded to the transducer and forming a backing element.

Fig. 3B schematically illustrates a good impedance match between the piezoelectric transducer and the component joined to the transducer and forming the backing element.

Fig. 4 schematically shows a test bench aimed at determining the propagation speed of sound waves through different materials.

Fig. 5A and 5B show detection echoes obtained using the test bench schematically shown in fig. 4 in the presence and absence of a sample under investigation, respectively.

Fig. 6A shows the propagation velocity (or wave velocity) of an acoustic wave (y-axis-unit m.s ^-1) as a function of the volume fraction of the pores (x-axis-%).

Fig. 6B shows the acoustic impedance (y-axis-in rayleigh x 10 ⁷) of a porous stainless steel material as a function of the volume fraction of the pores (x-axis-%).

Fig. 6C shows the linear attenuation coefficient (y-axis-in dB/mm) as a function of the volume fraction of the pores (x-axis-%).

Detailed Description

Fig. 2A shows a first embodiment of a transducer 1 according to the invention. As described in connection with the prior art, the transducer comprises a transducer 10 made of piezoelectric material, the transducer 10 being interposed between a front electrode 11 and a rear electrode 12. An assembly formed by the piezoelectric transducer 10, the front electrode and the rear electrode is arranged in the housing 2 comprising the aperture 15. The front electrode 11 is located between the aperture 15 and the piezoelectric transducer 10. The device preferably comprises an acoustic impedance matching plate 14, the acoustic impedance matching plate 14 being interposed between the front electrode 11 and the orifice 15. The impedance matching plate is formed of, for example, aluminum.

The transducer is connected to the circuit 20 such that an alternating voltage can be applied between the front and rear electrodes or measured between the front and rear electrodes. As described in connection with the prior art, under the application of a brief alternating voltage, sound waves EW are emitted through the orifice 15 and propagate through the surrounding medium 3. The surrounding medium may in particular be a liquid or a solid. It may be water, a liquid material or a solid material. Under the influence of the received sound wave RW, an alternating electrical signal is detected by the circuit 20, the amplitude of which corresponds to the amplitude of the vibration of the piezoelectric transducer under the influence of the received sound wave RW.

The piezoelectric transducer 10 may take the form of a disc having a thickness of 1mm and a diameter of 5mm to 50 mm. The materials used are compatible with use at high temperatures, for example 200 ℃ to 700 ℃ and even higher, for example exceeding 1000 ℃. The piezoelectric material may be lithium niobate (LiNbO ₃). The resonant frequency of the piezoelectric transducer may be from hundreds of kHz to several MHz, for example 4MHz or 5MHz. The thickness of each electrode may be of the order of 1 mm. Each electrode may take the form of a disc having a diameter corresponding to the diameter of the piezoelectric transducer 10.

The circuit 20 is connected to a central unit 30, which central unit 30 is configured to control the circuit when the transducer is operated in a transmitting mode and/or to analyze the voltage detected between the electrodes when the transducer is operated in a receiving mode.

The transducer 1 comprises a rear part 13 against the rear electrode 12. The rear component is intended to form a "backing element" with respect to the piezoelectric transducer 10. As mentioned in the prior art, this is to attenuate the echo of the sound wave emitted to the rear of the piezoelectric transducer. The backing element preferably has a thickness of more than 5mm, even 10mm. The thickness may be 10mm to 100mm, for example 40mm.

An important aspect of the present invention is that the backing element 13 is formed of a porous metal material (pure metal or metal alloy) having a melting point higher than 200 ℃, preferably higher than 600 ℃ or 700 ℃, preferably higher than 1000 ℃. The porous metal material may be, for example, steel (e.g., stainless steel), aluminum, or a metal selected from Ni, fe, pd, ag, au, cu, pd, al. The porous metal material may be a metal alloy, such as bronze or brass. The backing elements may be made of the same material as the rear electrode, which facilitates their engagement.

The metallic material, in particular stainless steel, has the advantage of good corrosion resistance, resistance to ionizing radiation (in particular neutrons or gamma radiation), which makes it compatible with use in nuclear power plants.

When the backing element is formed of the same material as the rear electrode (in this case a conductive metal), the stage of bonding the backing element to the electrode is avoided. In contrast, in the case where the backing member is made of ceramic, an adhesive or welding must be used to join the backing member to the electrode. Such joints may not be durable, particularly if the transducer is subjected to high temperature gradients. In particular, the respective coefficients of thermal expansion of the ceramic and metal electrodes are typically different from each other. This results in degradation of the bond over time, particularly in the event of repeated exposure to high temperature gradients.

The material forming the piezoelectric transducer 10 has a curie temperature above which the piezoelectric behavior of the piezoelectric transducer 10 is considered to disappear. Preferably, the metallic material forming the backing element has a melting point higher than the curie temperature of the piezoelectric transducer. The curie temperature of lithium niobate is higher than 1100 ℃.

The use of conductive metallic materials is advantageous. Fig. 2B schematically illustrates an embodiment in which the backing element and the rear electrode form the same part. Thus, the rear electrode is formed of an electrically conductive porous metal material. Such an embodiment is particularly advantageous because it minimizes the number of parts forming the transducer and simplifies manufacturing, particularly by avoiding the need to bond a backing element to the electrode.

The backing element 13 is configured to maximize transmission of the vibration waves generated by the piezoelectric transducer and minimize reflection of the waves. Fig. 3A and 3B show a configuration as shown in fig. 2B, in which the piezoelectric transducer 10 is arranged in direct contact with the backing element 13, the backing element 13 serving as a rear electrode. At the interface between the piezoelectric transducer 10 and the backing element 13, a transmission coefficient T and a reflection coefficient R may be defined. The transmission coefficient T corresponds to the ratio between:

The amplitude of the wave propagating from the piezoelectric transducer 10 and incident on the backing element 13 (referred to as the incident wave), this amplitude being denoted a _i in fig. 3A and 3B;

The amplitude of the transmitted wave a _t, the transmitted wave a _t corresponding to the portion of the incident wave propagating through the backing element.

The reflection coefficient R corresponds to the ratio between:

-the amplitude of the incident wave a _i;

The amplitude of the reflected wave a _r, the reflected wave a _r corresponding to the portion of the incident wave that is reflected by the backing element 13 and propagates to the piezoelectric transducer 10.

If Z ₁ and Z ₂ represent acoustic impedances of the piezoelectric transducer 10 and the backing element 13, respectively, the coefficients R and T are:

Fig. 3A schematically shows a configuration in which the absolute value of the reflection coefficient is high and the absolute value of the transmission coefficient is low. This is a non-matching configuration in that reflection of the incident wave at the interface between the piezoelectric transducer 10 and the backing element 13 produces an echo that increases the duration of the acoustic wave transmitted or received by the transducer. This results in a time degradation of the measurement, since the time of transmission or reception of the wave is determined with degraded accuracy. When the transducer is used for ranging purposes, the spatial resolution of the measurement may degrade.

Fig. 3B schematically shows a configuration in which the reflection coefficient is close to 0 and the transmission coefficient is close to 1, which corresponds to an ideal case. Echo formation at the interface between the piezoelectric transducer 10 and the backing element 13 is weak. This results in a shorter emitted (or detected) sound wave, thereby improving the time resolution of the measurement. The present invention aims to more closely approach this configuration. Expression (3) shows that if the acoustic impedances of two adjacent media are close to each other, or in other wordsSuch a configuration is obtained.

The acoustic impedance of the piezoelectric transducer 10 is typically tens of megarayls, typically 25 to 40 megarayls (mega rayl), compared to the acoustic impedance of air (430 rayls) or water (1.5 megarayls). The configuration shown in fig. 3A corresponds to an interface between the piezoelectric transducer and air. The configuration shown in fig. 3B corresponds to the interface between the piezoelectric transducer and the porous metal backing element.

In addition to having a transmission coefficient close to 1, the porous metal backing element must attenuate the transmitted sound waves.

The acoustic impedance and attenuation of the backing element 13 is controlled by the size and volume fraction of the air filled pores. Various experimental tests have been performed to define a range of sizes and volume fractions of pores that allow transmission coefficients approaching 1 and sufficient attenuation to be obtained. It should be noted that the size and volume fraction of the pores affects the impedance depending on the materials used. The values obtained under one material cannot be transferred to another material.

The key parameter is the speed of sound propagation c from which the acoustic impedance Z can be calculated using the following expression:

Where ρ is the density of the metallic material, expressed in kg.m ^-3, and c is expressed in m.s ^-1.

Fig. 4 schematically shows a test bench 100 for detecting a return sound (echometry). A metal sample 103 of different porosity is placed between the acoustic transmitter 101 and the acoustic receiver 102. The whole is immersed in water 104.

Fig. 5A shows pulses received by acoustic receiver 102 without a sample between the transmitter and the receiver. t corresponds to the detection time of the acoustic wave. The y-axis corresponds to amplitude and the x-axis corresponds to time. The propagation of sound waves through the water 104 is represented by the dashed arrows.

Fig. 5B shows the pulses received by the acoustic receiver 102 after the sample 103 of thickness e has been placed between the transmitter and the receiver. The y-axis corresponds to amplitude and the x-axis corresponds to time. The propagation of sound waves through the water 104 is represented by solid arrows. Considering the interface formed by the sample 103, two pulses are detected, the first pulse at time t ₁ corresponding to a wave that has propagated through the sample without reflection. Time t ₁ is earlier than time t with respect to the time of emission of the acoustic wave by emitter 101, because the propagation velocity of the acoustic wave in sample 103 is greater than the propagation velocity in water 104.

Based on these measurements, the acoustic propagation velocity (or wave velocity) c may be defined as follows:

-based on The expression is used:

in (5), c _w represents the sound propagation velocity in water.

Samples of 316L stainless steel were tested with different thickness e (5 mm or 10 mm), volume fractions with various porosities (25% to 53%) and various average pore sizes (average diameter of 2 μm to 60 μm). The following table shows the characteristics of the samples tested. Each sample 103 takes the form of a plate having dimensions of 50mm by 50 mm.

The volume fraction of porosity of each sample was determined by measuring the mass per unit volume. The average pore size was determined by light microscopy. Table 1 shows the main characteristics of the tested samples.

TABLE 1

Fig. 6A shows the propagation velocity of the acoustic wave determined using expression (6) according to the volume fraction change of the porosity. In fig. 6A, the y-axis corresponds to the wave velocity (unit m.s ^-1), and the x-axis corresponds to the porosity (%).

Fig. 6B shows acoustic impedance calculated using (4) based on knowledge of the density ρ of the material under test and the wave velocity obtained using expression (6). The y-axis corresponds to the velocity (in m/s) and the x-axis corresponds to the volume fraction (%) of pores.

The linear attenuation coefficient a (in dB/mm) may also be determined from the maximum amplitude of the wave detected by the receiver 102. The attenuation is measured by comparing the amplitudes of the pulses detected at times t ₁ and t ₂, respectively, as defined with reference to fig. 6B.

From the calculated attenuations Att _a and Att _b for each sample, a linear attenuation coefficient per millimeter (denoted as α) was determined taking into account the following:

when considering Att _a, the thickness e of the sample;

When considering Att _b, twice the thickness e of the sample;

Fig. 6C shows the attenuation coefficient α obtained using expression (7). The y-axis corresponds to attenuation (units dB/mm) and the x-axis corresponds to the volume fraction (%) of pores.

In fig. 6A and 6C, each point corresponds to one measurement value. The function obtained by interpolation of each measurement point has also been plotted in dashed lines. In fig. 6A and 6B, the interpolation is linear. In fig. 6C, the function obtained by interpolation is a polynomial.

Fig. 6B defines a range of volume fractions of porosity (expressed in%) where the impedance is sufficiently close to the impedance of the piezoelectric material, i.e., in the range of 10 megarayls to 40 megarayls. This corresponds to a volume fraction of porosity of less than 50%, according to fig. 6B.

Fig. 6C defines a range of volume fractions of porosity (expressed in%) where attenuation is sufficient. Sufficient attenuation means that the attenuation coefficient α is greater than or equal to 1dB/mm. According to fig. 6C, this corresponds to a volume fraction of porosity of greater than 25%.

The range of optimal porosities is such that:

the impedance of the porous metal material forming the backing element is sufficiently high (close to the impedance of the piezoelectric material), it being noted that the impedance decreases with increasing volume fraction of porosity, see fig. 6B;

The attenuation is high enough, it being noted that the linear attenuation coefficient α increases with increasing volume fraction of porosity, see fig. 6C.

From the foregoing, it can be appreciated that the attenuation effect is due to the presence of voids, while the impedance matching effect is due to metal. The porosity characteristics of the metal are thus a result of the volume fraction tradeoff of pores.

For 316L type stainless steel, the optimal range of volume fraction of porosity is 25% to 50% considering that the pore size (i.e., average diameter) is 2 μm to 60 μm.

The optimal range of volume fractions of porosity may be different for another material. In general, the metallic material forming the backing element 13 of the transducer is characterized by:

-average pore diameter less than 500 μm, and less than 200 μm or less than 100 μm;

-and/or the volume fraction of porosity is 20% to 60%, preferably 25% to 50%, more preferably 25% to 40%.

In the embodiments described above, the piezoelectric material forming the transducer is made of lithium niobate. Other piezoelectric materials suitable for high temperature environments may be used, such as barium titanate (BaTiO ₃), bismuth titanate (BiTiO ₃) and derivatives thereof (e.g., added sodium), aluminum nitride (AlN), lanthanum salts (lanthanum oxide, gallium oxide, and tantalum oxide).

Transducers according to the invention will be useful for any high temperature application for non-destructive testing or diagnosis or ranging or obstacle detection or flow measurement purposes.

Claims

1. An acoustic transducer device, comprising:

- a piezoelectric converter (10) formed of a piezoelectric material, the piezoelectric converter (10) being interposed between a front electrode (11) and a rear electrode (12);

- a housing (2) which accommodates the piezoelectric converter, the front electrode and the rear electrode;

- a front aperture (15) formed in the housing and arranged such that the front electrode is arranged between the piezoelectric material and the front aperture (15);

The device is configured to emit acoustic waves (EW) toward the front aperture or to detect acoustic waves (RW) propagating from the front aperture;

The device comprises a rear part (13) forming a backing element of the device, the device being characterized in that the rear part is a porous metal material having a melting point above 200° C., the device being such that:

- said rear component forms said rear electrode;

- The thickness of the rear component is greater than 5 mm.

2. A device according to any preceding claim, wherein the thickness of the rear component is greater than 10 mm.

3. The device according to claim 1 or 2, wherein the melting point of the metal material is higher than 600°C.

4. An apparatus according to any preceding claim, wherein:

- the piezoelectric material has a Curie temperature;

- The melting point of the metal material is higher than the Curie temperature of the piezoelectric material.

5. The device of claim 4, wherein the Curie temperature of the piezoelectric material is greater than 1000°C.

6. A device according to any preceding claim, wherein the volume fraction of pores in the porous metal material is from 20% to 60%, or from 25% to 50%, or from 25% to 40%.

7. A device according to any preceding claim, wherein the average size of the pores is less than 100 μm, said average size corresponding to the average diameter of each pore.

8. A device according to any preceding claim, wherein the piezoelectric material is selected from lithium niobate and barium titanate.

9. The device according to any preceding claim, wherein the metallic material comprises at least one element selected from Ni, Fe, Pd, Ag, Au, Cu, Pd, Al.

10. An apparatus according to any preceding claim, wherein the metallic material is a stainless steel alloy.

11. Use of a device according to any preceding claim for emitting or receiving sound waves (EW, RW), the emitted or received sound waves propagating through the orifice (15), the device being arranged in a medium (3) at a temperature above 200°C.